Need to track down this carb issue

A while back I picked up a lightly used Holley Street Avenger 670. Put a kit in it to be safe. Long story short, won't idle. Blacked out the plugs from a few minutes of running, and will only stay running by nursing the choke and/or throttle.

Got my hands on a used Edelbrock 750. Fires right up.

Spoke with Holley, the guy didn't seem super happy to talk to me. Says it's a vacuum leak or idle circuit clogged. This carb is very very clean. I pulled it apart again, checked the idle passages, all clear from what I can tell. So it seems that if there's a vacuum leak, it's internal to the carb. I'm kind of out of ideas. Maybe the new power valve is bad?